A typical inkjet recording device includes an inkjet head that discharges minute droplets of ink to form an image on a recording medium. The inkjet head includes nozzles aligned at a predetermined pitch and discharging ink at controlled timings depending on the resolution of the image to be formed and/or the rate of image formation.
Such an inkjet head is typically of a piezoelectric or thermal inkjet type that discharges ink from the tips of the nozzles through pressurization of the ink in the ink flow paths. An increase in image resolution has resulted in a decrease in the volume of ink per discharge. The discharge of such a minute volume of ink requires a decrease in dimension of the nozzle orifices and the time of pressurization. Unfortunately, a decrease in dimension of the cross-section of the nozzles orifices enhances the influence of friction occurring between the ink and nozzle walls and viscosity and surface tension of the ink, which prevents the ink in the ink flow paths from stably separating and being discharged as droplets having a desired volume.
PTL 1 discloses an electrostatic attraction type inkjet head that applies a voltage to the tips of minute nozzles to attract charged ink and discharge the ink in minute volumes.
The ink is supplied to the nozzles from a common ink chamber for the nozzles through ink leading paths (inlets) for the nozzles and pressure chambers provided for pressurization of the ink. The ink pressure varies depending on a natural frequency (resonant frequency) determined in accordance with the dimensions of the inlets, pressure chambers, and nozzles (length in particular), and the density and viscosity of the ink. The fluid level (meniscus) of the ink shifts inward or outward from the orifice edges of the nozzles in accordance with the cycle of natural frequency. The natural oscillation includes an inherent mode (first-order mode) oscillation on the overall structure of the inlets, pressure chambers, and nozzles, and one or more other inherent modes on a part of the structure, depending on the differences in structure (shape) of the inlets, pressure chambers, and nozzles.
Among the inherent modes, the mode of a portion including the nozzles involves the extrusion of ink at the orifice edges of the nozzles and is a second-order mode having a frequency higher than the natural frequency of the first-order mode. PTL 2 discloses a technique of applying pressure to ink in pressure chambers at a cycle corresponding to the natural frequency of the second-order mode, to reduce the time of ink discharge and thereby the volume of the discharged ink.